"Debug Mode" world causes error on world list

The bug

If you created a "Debug Mode" world, then close the world and return to the singleplayer world list, the follow error is logged:

WorldGenSettings: Input does not contain a key [type]: MapLike[{}] missed input: {"minecraft:overworld":{generator:{},type:"minecraft:overworld"}}

Reproduction steps

1. Create a "World Type: Debug Mode" world
2. Close the world and return to the world selection list
   An error is logged